This application claims the priority of Japanese patent application Serial No. 11-310063 filed on Oct. 29, 1999.
The present invention relates to a shadow mask to be usable for high-definition color Braun tubes (cathode ray tubes) such as color television display, computer display or the like, and more particularly, to a shadow mask which is free from color mismatching to be caused by thermal expansion of the shadow mask and has an excellent accuracy in dimension of an opening portion (aperture).
Conventionally, a shadow mask has been used for color Braun tubes such as color television display, computer display or the like. The shadow mask is located to a predetermined portion in the color Braun tube and comprises opening portions through which electron beam is radiated to a fluorescent material layer (phosphor layer) formed to an inner surface of the Braun tube.
The shadow mask takes various structures for the opening portions so that:
(a) a number of small and round through holes (circular apertures) are formed;
(b) a number of small through holes (apertures) each having a rectangular-shape are formed; and
(c) a number of slits are provided so as to align with each other.
The shadow masks formed with the opening portions having structures specified in (a) and (b) are hereinlater referred to as xe2x80x9cpress-type shadow maskxe2x80x9d for the sake of convenience and usually manufactured by press-molding (forming) method so as to match with an inner shape of the Braun tube. On the other hand, the shadow mask formed with the opening portion having a structure specified in (c) is hereinlater referred to as xe2x80x9cexpansion-type shadow maskxe2x80x9d for the sake of convenience and is called an aperture grill. The expansion-type shadow mask is usually manufactured by fixing a mask body to a rigid steel frame while the slits are expanded in a longitudinal direction thereof.
As shown in FIG. 2, the through holes and slits are generally formed by simultaneously etching both surfaces of a mask plate member, and each of the through holes consists of a small hole portion (aperture) 4 formed to one side 2 of the mask plate member to which electron beam incidents and a large hole portion 6 formed to the other side opposing to a side of a fluorescent surface formed to the inside of the Braun tube. A diameter of the through hole or width of the slit through which the electron beam passes is determined by the small hole portion 4, the large hole portion 5 and an intersecting portion of both the small and large hole portions.
In the Braun tube comprising the shadow mask, a part of electron beams radiated from an electron gun collide with a surface of the shadow mask without passing through the through holes and the slits. Therefore, the shadow mask is heated through the collision of the electron beams.
In this regard, since the conventional shadow mask is formed of a low-carbon steel plate having a large thermal expansion coefficient, the shadow mask is liable to cause thermal expansion due to the heat generation based on the collision of the electron beams. Accordingly, there has been caused phenomena such that a displacement and deformation of the through holes are disadvantageously caused and slacks are arisen at the slit-portions to thereby cause the disturbance in slit interval.
Such phenomena lead to the displacement of the electron beams to be attained to the fluorescent material layer (phosphor layer) formed inside the Braun tube, so that there has been posed a problem such that color mismatching is caused in an image reproduced on the Braun tube.
In particular, in these days, a color television set and computer display unit or the like have been required to display an image having a high resolution, so that it has been also required for the shadow mask to decrease a pitch of the through hole or the slit. However, such demand has not been attained up to now, and there is a fear that the above color mismatching in the image based on the thermal expansion will be more remarkable.
To cope with these problems, for the purpose of suppressing the thermal expansion of the metal material, it has been tried to use a metal material having a small thermal expansion coefficient. For example, some of the shadow masks are formed by using iron-nickel alloy or iron-nickel-cobalt alloy. Such shadow masks have an advantage of that the thermal expansion hardly occurs even if the heat generation is took place by the collision of the electron beams.
However, inherently, the iron-nickel alloy plate and the iron-nickel-cobalt alloy plate have a relatively large crystal grain in comparison with that of a conventional low-carbon steel plate which has been widely used for the alloy plate. Accordingly, there has been posed a problem such that it was difficult to smoothly perform an etching treatment for the alloy plate so as to obtain a fine through hole or slit excellent in the dimensional accuracy.
In such the etching treatment, the etching function of forming the through hole or the slit progresses simultaneously not only in a depth (thickness) direction of the alloy plate but also in a width (longitudinal or lateral) direction of a plate member. In particular, in a case where the etching function easily progresses, the small hole portion and the large hole portion each having an excessively large size are disadvantageously formed, so that the respectively adjacent through holes or slits are joined together at the large hole portions. Therefore, it was very difficult to decrease the pitch between the adjacent opening holes. Accordingly, there was a fear such that it was difficult to manufacture a shadow mask having a reduced pitch required for finely forming the adjacent opening holes. In addition, since a hole diameter of the large hole portion becomes excessively large, there was a fear that the strength of the shadow mask itself becomes lowered, thus being disadvantageous.
Further, there was also a fear that the lowering of the strength of the shadow mask itself based on the excessive progressing of the etching function would induce a deformation of a shape of the shadow mask through impacts, shocks or vibrations that were inevitably caused at the time when the shadow mask was transferred during the manufacturing process thereof or the shadow mask was attached to the Braun tube, and further, the Braun tube was transferred or transported.
In particular, in the case of a shadow mask for a large-sized display type or a flat-type Braun tube, it was required to form a large hole portion having a large size so as to reduce the pitch of the through hole or the slit and so as not to obstruct the passing through of the electron beam, thus providing a problem that it was difficult to manufacture a shadow mask excellent in the dimensional accuracy and a problem that the deformation was liable to cause due to the lowering of the strength of the shadow mask.
To cope with the above problems, there has been adopted a method in which a shadow mask excellent in the dimensional accuracy of the through hole or slit is obtained by reducing a crystal grain size in a metal material for making a wall surface of the opening portion smooth after the completion of the etching treatment.
However, when the crystal grain size in the metal material is reduced, an etching speed is disadvantageously lowered, thus also providing a problem that a longer time for performing the etching treatment is required than that in the case of using the conventional low-carbon steel plate.
In addition, there was also an actual situation that the through hole or slit is hardly formed so as to have a predetermined aimed shape even if only the crystal grain size is reduced. Particularly, in the case of a press-type shadow mask to which the through holes each having a complete round-shape are formed, there was posed a problem that the shape of the through hole is difficult to be formed so as to have a complete round shape.
As another countermeasure, there has been well known a shadow mask composed of an iron-nickel alloy plate in which a composition or surface cleanliness of a metal material is properly adjusted or a crystal face orientation of the metal material is controlled to a specific direction. However, any of the above countermeasures has not attained to a level capable of solving the problems mentioned above.
An object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a shadow mask for a color Braun tube capable of preventing a diameter of a large hole portion from being excessively enlarged by improving a hole shape of an opening portion.
This and other objects can be achieved according to the present invention by providing a shadow mask composed of an iron-based alloy plate containing 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt, wherein the iron-based alloy has a crystal grain size number of 10 or more and 12 or less, has a crystal grain size of 50 xcexcm or less in a cross section in a direction parallel and normal to a rolling direction of the iron-based alloy plate, and has an average crystal grain size of 30 xcexcm or less in a cross section in a direction parallel to the rolling direction of the iron-based alloy plate.
The iron-based alloy has a thermal expansion coefficient of approximately 4.0xc3x9710xe2x88x926/xc2x0 C.
More in detail, there is provided a shadow mask for a Braun tube comprising:
a frame member;
a mask body in shape of plate made of an iron-based alloy material; and
an opening portion formed to the mask body,
wherein the iron-based alloy material contains 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt, the iron-based alloy has a crystal grain size number of 10 or more and 12 or less, has a crystal grain size of 50 xcexcm or less in a cross section in a direction parallel and normal to a rolling direction of the iron-based alloy, and has an average crystal grain size of 30 xcexcm or less in a cross section in a direction parallel to the rolling direction of the iron-based alloy.
According to the present invention of the characters mentioned above, since the iron-based alloy plate containing 31.0-38.0 weight % of nickel and 1.0-6.5 weight % of cobalt has a crystal grain size number of 10 or more and 12 or less, and the iron-based alloy is composed of fine crystal grains by specifying the crystal grain size so as to have a crystal grain size of 50 xcexcm or less in a cross section in a direction parallel and normal to a rolling direction of the iron-based alloy plate and an average crystal grain size of 30 xcexcm or less in a cross section in a direction parallel to the rolling direction of the iron-based alloy plate, so that an etching treatment can be performed with a high accuracy at the time of manufacturing the shadow mask.
As a result, a shadow mask being free from unevenness and having an improved shape, of the opening portion for a color Braun tube can be effectively manufactured. In addition, it becomes possible to prevent the large hole portion from being excessively etched, so that the reduction of pitch for providing the opening holes in accordance with a trend toward high definition can be realized, and the lowering the strength of the shadow mask can be prevented. A Braun tube provided with the shadow mask of the present invention can sufficiently cope with the trend toward a high-definition, a quality of an image reproduced on a television can be further improved.